Magnetic resonance imaging (MRI) measures tissue-specific responses to a radio frequency (RF) stimulus in a strong static main magnetic field (BO). Specifically, the magnetization of tissue is aligned with BO. An initial RF pulse tips the magnetization out of this alignment and rotates with a tissue-specific RF frequency, resulting in a signal that is picked up with a receiver coil. Additional magnetic field gradient pulses (G) are used to spatially encode the RF signal that in turn is used to construct an image.
Before acquiring images, the MR scanner goes through an adjustment step, whereby the transmitter frequency is appropriately tuned so as to stimulate only desired tissue, excluding undesired signal from fat. If fat signal dominates in the field-of-view (FOV), as might happen in obese subjects with large amounts of subcutaneous fat, the scanner may not be appropriately tuned, thereby degrading the image quality.
MRI relies on a very homogeneous static magnetic field. Unfortunately, the magnetic field homogeneity of a clinical MRI scanner is degraded in the presence of the human body. In addition, static magnetic field homogeneity is degraded by interfaces between tissues of differing magnetic susceptibility (e.g., lung and liver), with this effect being highly patient dependent. Although the static magnetic field homogeneity can be improved by the process of shimming, the need to shim over a large FOV limits the efficacy of this technique. In addition to the reliance on static magnetic field homogeneity, MRI also requires that there is negligible motion from the time of the initial RF pulse through the application of gradient pulses and the reception of signal; this is because motion disrupts the spatial encoding introduced by the gradients, resulting in artifacts in the final images.
Image quality in MRI is affected not only by motion of the organ of interest, but also the motion of other organs within the FOV. For example, while imaging the thoracic spine, which is stationary, cardiac and breathing motion can degrade image quality in the spine region. Currently, MR images are generally acquired such that the FOV is large enough to cover all tissues within the slice of interest. In some attempts to reduce the FOV, efforts have focused on specially designed RF pulses to excite a small FOV covering the region of interest.
This document describes a method and system for minimizing artifacts from features in the slice that are outside of the region of interest.